The present invention relates to techniques for drilling oil, gas, water, or geothermal wells and the like. More precisely, the invention relates to cementing compositions that are particularly adapted to cementing zones which are subjected to extreme dynamic stresses.
In general, a well whose depth exceeds a few hundred meters is cased, and the annulus between the underground formation and the casing is cemented over all or part of its length. The essential function of cementing is to eliminate fluid interchange between the various layers of formation through which the borehole passes and to control ingress of fluid into the well, in particular by limiting ingress of water. In production zones, the casing is perforated, as indeed are the cement and the formation over a depth of several centimeters.
The cement placed in the annulus of an oil well is subjected to numerous stresses throughout the lifetime of the well. The pressure inside the casing can increase or decrease because of a change in the fluid filling the casing or because additional pressure is applied to the well, such as when drilling fluid is replaced by completion fluid or by fluid for a stimulation operation. A change in temperature also gives rise to stress on the cement, at least during the transient period, which precedes temperature equilibrium being reached between the steel and the cement. In most of the above cases, the stress process is sufficiently slow to be dealt with as though it was a static process.
Nevertheless, the cement can be stressed in other ways, which are dynamic, either, because they take place over a very short period of time or because they are periodic, or at least repetitive. Not only does making the perforations admit excess pressures of several hundreds of bars to the inside of the well, which are dissipated in the form of shock waves, but in addition it gives rise to shock when the projectile penetrates the cement. This shock subjects the zone surrounding the hole to large forces over a length of several meters.
Another process which is now very common in oil well operations and that also gives rise to dynamic stresses in the cement is opening a window in casing that has already been cemented for the purpose of creating a lateral well. The steel is milled over a height of several meters and then the lateral hole is bored, subjecting the cement to shock and vibration which generally damages it in irremediable manner.
An object of the present invention is to provide novel formulations, specifically for cementing regions of oil wells or the like that are subjected to extreme dynamic stresses, such as zones that are punctured and junctions with side well branches.
In the fields of building and civil engineering, it is well known to reinforce cement with fibers. By way of example, mention can be made of asbestos fibers or glass fibers for reinforcing thin materials, specifically materials in plate form. Polymer fibers made of polypropylene or nylon, or indeed carbon fibers for applications having particularly severe specifications, are also commonly used, specifically in sprayed concrete and facework treatment techniques. EP-576,401 discloses mortar composition which comprises an hydraulic cement, a mixture of admixtures, a continuum of particles between 0,1 xcexc and 10 mm and cast iron fibers having a length between 10 and 40 mm and preferably between 20 and 30 mm. The upper limit of workability of the fluid mortar composition is said to be no more than 42 kg/m3.
In the field of oil industry cements, various publications have suggested using mineral fibers (U.S. Pat. No. 5,421,409), in particular asbestos fibers (U.S. Pat. No. 1,010,253), or other fibers commonly used in building or civil engineering (U.S. Pat. No. 1,006,713). In a May 1995 article VAN VLIET, VAN KLEEF, SMITH, PLOMPEN, KUIJEVENHOVEN, QUARESMA, and RAITURKAR VLIET, et al. have suggested using cements that include synthetic fibers, in particular fibers of polypropylene or nylon for sheathing oil wells. That article also suggests using the same synthetic fibers for applications such as making plugs or as a plugging material against circulation losses.
An object of the present invention is to obtain oil industry cements that are reinforced with fibers and that have improved properties of tensile strength and impact resistance. According to the invention, this problem is solved by adding fibers of amorphous cast metal to the cement slurry.
Amorphous cast metal fibers are known, e.g. from U.S. Pat. No. 4,520,859, and they are obtained by casting a fine ribbon of molten metal on a cold drum. Rapid cooling prevents crystallization, so the metal solidifies in the form of an amorphous material. The longest fibers give the best results from the point of view of tensile strength. It is thus preferable to use fibers that are at least 5 mm long. In addition, given that the width of the annulus to be cemented in an oil well is generally about 30 mm, the length of the fibers should not exceed 15 mm and preferably be between 5 and 10 mm.
Amorphous cast metal fibers are added to the cement slurry of the invention at a concentration of 3% to 15% by weight of fibers relative to the weight of cement, i.e. typically with fiber concentrations in the slurry of the order of 50 kg/m3 to 200 kg/m3, and preferably lying in the range 75 kg/m3 to 150 kg/m3.
In oil well cementing, non-homogeneous cement columns are not acceptable, particularly to when the wellbore is highly deviated. To avoid particle settling with such high concentrations of fibers, the yield stress xcfx84y, i.e. the minimum stress to which the slurry must be submitted to flow, as defined using the Bingham Plastic rheogical model must be of at least 7 Pa and to remain pumpable, no more than 25 Pa, at the temperature of pumping. Preferred slurries have a yield stress ranging between 10 and 15 Pa. Anti-settling additives, such as cellulosic derivatives (hydroethylcellulose) or mixtures of biopolymer and silica flour for example are added to the slurry to adjust the yield stress.
At the higher concentrations, it is preferable to use mixtures of short fibers or mixtures of short and long fibers which mixtures present the advantages of short fibers from the point of pumpability.
As shown in particular in French patent application 97 11821 filed on Sep. 23, 1997 in the name of the Applicant, the risk of a cement sheath breaking due to an increase in the pressure or the temperature in a well is directly related to the tensile strength of the cement, and said risk is attenuated when the ratio of the tensile strength of the cement over its Young""s modulus increases. It is recalled that the more flexible a material, the smaller its Young""s modulus.
When an increase in temperature or pressure persists, sheath damage can also be caused by radial stresses acting on the sheath, which stresses are in compression.
From that work, it appears that any additive seeking to improve the mechanical properties of a cement sheath must give rise to a cement that has both improved tensile strength and improved strength in compression, a high degree of flexibility, and as large as possible a ratio of cement strength (in traction and in compression) over its Young""s modulus. It is particularly noticeable that this is indeed the case with systems having amorphous cast metal fibers.
The amorphous cast metal fibers can be added to conventional slurries based on Portland cement, and also to special cementing slurries, e.g. a slurry based on aluminous cement.
The cement slurries according to the present invention are particularly suitable for cementing multi-lateral wells (due to their good impact resistance). There are also particularly adapted for borehole lining applications when the cement has to be drilled which fibers prevent cement from falling apart and provides a good impact resistance to the shock generated by the drill bits.
The present invention is illustrated by the following examples.
Fiber Selection
The influence of various fibers on the mechanical properties of a cement slurry has been studied on systems obtained under normal conditions of temperature and pressure (laboratory temperature and pressure).